\Tro"choid\, n. [Gr. ? a wheel + -oid; cf. F.
trocho["i]de. See {Troche}.] (Geom.)
The curve described by any point in a wheel rolling on a
line; a cycloid; a roulette; in general, the curve described
by any point fixedly connected with a moving curve while the
moving curve rolls without slipping on a second fixed curve,
the curves all being in one plane. Cycloids, epicycloids,
hypocycloids, cardioids, etc., are all trochoids.

\Tro"choid\, a.
1. (Anat.) Admitting of rotation on an axis; -- sometimes
   applied to a pivot joint like that between the atlas and
   axis in the vertebral column.
2. (Zo["o]l.) Top-shaped; having a flat base and conical
   spire; -- said of certain shells.
3. (Zo["o]l.) Of or pertaining to the genus Trochus or family
   {Trochid[ae]}.